Method for holographic data retrieval by quadrature homodyne detection

ABSTRACT

Systems and methods are provided for recovering data in a holographic memory system. One embodiment of these systems and methods uses homodyne detection to introduce a local oscillator beam into a reconstructed data beam of the recovered hologram. An image of the combined beam comprising the reconstructed data beam and local oscillator beam may be processed to obtain contrast level information for the pixels of the detected image. This contrast level information may then be used to obtain an increased contrast image of the recovered hologram, which may increase the SNR of the recovered data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application makes reference to and claims the benefit of thefollowing co-pending U.S. Provisional Patent Application No. 60/738,597filed Nov. 25, 2005. The entire disclosure and contents of the foregoingProvisional Application is hereby incorporated by reference. Thisapplication also makes reference to the following co-pending U.S. patentapplications. The first application is U.S. application Ser. No.10/879,847, entitled “Method and System for Equalizing Holographic DataPages,” filed Jun. 28, 2004. The second application is U.S. applicationSer. No. 11/069,007, entitled “Processing Data Pixels in a HolographicData Storage System,” filed Feb. 28, 2005. The entire disclosure andcontents of the foregoing U.S. patent applications are herebyincorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention relates broadly to holographic memory systems, andmore particularly to methods and systems for holographic data retrieval.

2. Related Art

Developers of information storage devices continue to seek increasedstorage capacity. As part of this development, memory systems employingholographic optical techniques, referred to as holographic memorysystems, have been suggested as alternatives to conventional memorydevices.

Holographic memory systems may read/write data to/from a photosensitivestorage medium. When storing data, holographic memory system oftenrecord the data by storing a hologram of a 2-dimension array, commonlyreferred to as a “page,” where each element of the 2-D array is a singledata bit. This type of system is often referred to as “page-wise” memorysystem. Holographic memory systems may store the holograms as a patternof varying refractive index and/or absorption imprinted into the storagemedium.

Holographic systems may perform a data write (also referred to as a datarecord or data store operation, simply “write” operation herein) bycombining two coherent light beams, such as laser beams, at a particularpoint within the storage medium. Specifically, a data-encoded light beamis combined with a reference light beam to create an interferencepattern in the photosensitive storage medium. The interference patterninduces material alterations in the storage medium to form a hologram.

Holographically-stored data may be retrieved from the holographic memorysystem by performing a read (or reconstruction) of the stored data. Theread operation may be performed by projecting a reconstruction or probebeam into the storage medium at the same angle, wavelength, phase,position, etc., as the reference beam used to record the data, orcompensated equivalents thereof. The hologram and the reference beaminteract to reconstruct the data beam.

The reconstructed data beam may then be detected by a power-sensitivecamera and processed for delivery to an output device. This detectionmechanism may have several limitations. First, since hologramdiffraction efficiency is driven to the lowest possible level in orderto maximize the number of pages that may be stored, the read signals maybe weak and require long exposure times to detect. Secondly, the laserlight used to perform the read-out may be necessarily coherent, thusoptical noise sources such as scatter and ISI (intersymbol interference,or pixel-to-pixel crosstalk from blur) may mix coherently with thedesired optical signal, reducing signal quality when compared toadditive noise of the same power. As such, there may be a need toimproved the signal level of the detected hologram and improve thesignal to noise ratio.

SUMMARY

According to a first broad aspect of the present invention, there isprovided a method for use with a storage medium that holographicallystores information, the method comprising the following steps:

-   -   (a) generating a reconstructed data beam by directing a first        coherent light beam into a storage medium, wherein the first        coherent light beam reproduces a reference beam used to        holographically store information in the storage medium;    -   (b) obtaining a first image from a first combined beam produced        by combining at least a portion of the reconstructed data beam        with at least a portion of the second coherent light beam,        wherein the reconstructed data beam and the second coherent        light beam have a phase difference that is a first phase        difference;    -   (c) obtaining a second image from a second combined beam        produced by combining the reconstructed data beam with the        second coherent light beam, wherein the phase difference of the        reconstructed data beam and the second coherent light beam has        been adjusted to produce a second phase difference;    -   (d) processing the first image and second image to obtain first        image contrast information and second image contrast        information; and    -   (e) obtaining combined information from the first image and        second image using the first image contrast information and the        second image contrast information; and    -   (f) processing the combined information to obtain the        information holographically stored by the storage medium.

According to a second broad aspect of the present invention, there isprovided a system for use with a storage medium that holographicallystores information, the system comprising:

-   -   a light source which generates a first coherent light beam that        is a reproduction of a reference beam used in holographically        storing information in a storage medium and wherein the first        coherent beam generates a reconstructed data beam from the        storage medium;    -   a beam combiner which combines at least a portion of the        reconstructed data beam with at least a portion of a second        coherent light beam to produce a combined beam;    -   a camera which captures an image from the combined beam;    -   a phase retarder which causes a first phase difference and a        second phase difference between the reconstructed data beam and        the second coherent beam; and    -   a processor which (a) directs the phase retarder to cause the        first phase difference between the reconstructed data beam and        the second coherent beam to obtain a first image from the camera        of the combined beam when the reconstructed data beam and the        second coherent beam have the first phase difference; (b)        directs the phase retarder to cause the second phase difference        between the reconstructed data beam and the second coherent beam        to obtain a second image from the camera of the combined beam        when the reconstructed data beam and the second coherent beam        have the second phase difference; (c) processes the first image        and second image to obtain first image contrast information and        second image contrast information, (d) combines information from        the first image and second image using the first image contrast        information and the second image contrast information to obtain        combined information; and (e) processes the combined information        to obtain the information holographically stored by the storage        medium.

According to a third broad aspect of the present invention, there isprovided a system for use with a storage medium that holographicallystores information, the system comprising:

-   -   means for generating a reconstructed data beam by directing a        first coherent light beam into a storage medium, wherein the        first coherent light beam reproduces a reference beam used to        holographically store information in the storage medium;    -   means for obtaining a first image from a first combined beam        produced by combining at least a portion of the reconstructed        data beam with at least a portion of a second coherent light        beam, wherein the reconstructed data beam and the second        coherent light beam have a phase difference that is a first        phase difference;    -   means for obtaining a second image from a second combined beam        produced by combining the reconstructed data beam with the        second coherent light beam, wherein the phase difference of the        reconstructed data beam and the second coherent light beam has        been adjusted to produce a second phase difference;    -   means for processing the first image and second image to obtain        first image contrast information and second image contrast        information; and    -   means for obtaining combined information from the first image        and second image using the first image contrast information and        the second image contrast information; and    -   means for processing the combined information to obtain the        information holographically stored by the storage medium.

According to a fourth broad aspect of the present invention, there isprovided a method for use with a storage medium that holographicallystores information, the method comprising the following steps:

-   -   (a) generating a combined beam comprising a local oscillator        portion and a reference pattern portion;    -   (b) causing the combined beam to contact the storage medium to        thereby generate a reconstructed data beam;    -   (c) obtaining an image from the reconstructed data beam;    -   (d) processing the obtained image to provide image contrast        information; and    -   (e) modifying the local oscillator portion of the combined beam        based on the image contrast information.

According to a fifth broad aspect of the present invention, there isprovided a system for use with a storage medium that holographicallystores information, the system comprising:

-   -   a light source generating a first coherent light beam which        reproduces a reference beam used in holographically storing        information in a storage medium;    -   a spatial light modulator (SLM) which forms a combined beam from        the first coherent light beam, wherein the SLM comprises a        plurality of pixels which implement a format, wherein the format        comprises a local oscillator portion and a reference pattern        portion;    -   an optical steering subsystem which directs the first coherent        light beam towards the storage medium to generate a        reconstructed data beam;    -   a camera which obtains an image from the reconstructed data        beam;    -   a processor which processes the obtained image to provide image        contrast information and to direct the SLM to modify at least        one pixel in the local oscillator portion based on the image        contrast information.

According to a sixth broad aspect of the present invention, there isprovided a system for use with a storage medium that holographicallystores information, the system comprising:

-   -   means for generating a combined beam comprising a local        oscillator portion and a reference pattern portion;    -   means for causing the combined beam to contact the storage        medium to thereby generate a reconstructed data beam;    -   means for obtaining an image from the reconstructed data beam;    -   means for processing the obtained image to provide image        contrast information; and    -   means for modifying the local oscillator portion of the combined        beam based on the image contrast information.

According to a seventh broad aspect of the present invention, there isprovided a method for use with a storage medium that holographicallystores information, the method comprising the following steps:

-   -   (a) generating a reconstructed data beam by directing a first        coherent light beam into a storage medium, wherein the first        coherent light beam reproduces a reference beam used to        holographically store information in the storage medium;    -   (b) generating a local oscillator beam comprising a phase        modulation pattern;    -   (c) generating a combined beam comprising at least a portion of        the local oscillator beam and at least a portion of the        reconstructed data beam;    -   (d) obtaining an image from the combined beam;    -   (e) processing the obtained image to provide image contrast        information; and    -   (f) modifying the phase modulation pattern of the local        oscillator beam based on the image contrast information.

According to a eighth broad aspect of the present invention, there isprovided a system for use with a storage medium that holographicallystores information, the system comprising:

-   -   a light source generating a first coherent light beam which        reproduces a reference beam used in holographically storing        information in a storage medium and wherein the first coherent        beam generates a reconstructed data beam from the storage        medium;    -   an SLM which imparts a phase modulation pattern to a second        coherent beam;    -   a beam combiner which combines at least a portion of the        reconstructed data beam with at least a portion of a second        coherent beam;    -   a camera which obtains an image from the combined beam; and    -   a processor which processes the obtained image to provide image        contrast information and to direct the SLM to modify the phase        modulation pattern based on the image contrast information

According to a ninth broad aspect of the present invention, there isprovided a system for use with a storage medium that holographicallystores information, the system comprising:

-   -   means for generating a reconstructed data beam by directing a        first coherent light beam into a storage medium, wherein the        first coherent light beam reproduces a reference beam used to        holographically store information in the storage medium;    -   means for generating a local oscillator beam comprising a phase        modulation pattern;    -   means for generating a combined beam comprising at least a        portion of the local oscillator beam and at least a portion of        the reconstructed data beam;    -   means for obtaining an image from the combined beam;    -   means for processing the obtained image to provide image        contrast information; and    -   means for modifying the phase modulation pattern of the local        oscillator beam based on the image contrast information.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a simplified diagram of a holographic memory systemshowing one way for introducing a local oscillator beam, in accordancewith embodiments of the invention;

FIG. 2 illustrates exemplary signals where the local oscillator beam iscoherent and phase-locked in time and phase matched in space with thereconstruction data beam, in accordance with embodiments of theinvention;

FIG. 3 illustrates exemplary signals where the local oscillator beam andreconstructed data beam are 180 degrees out of phase in time, inaccordance with embodiments of the invention;

FIG. 4 illustrates exemplary signals where the local oscillator beam andreconstructed data beam are 90 degrees out of phase in time, inaccordance with embodiments of the invention;

FIG. 5 illustrates an exemplary P image of a detected hologram, inaccordance with embodiments of the invention;

FIG. 6 illustrates an exemplary Q image of a detected hologram, inaccordance with embodiments of the invention;

FIG. 7 provides an exemplary flow chart of a method for generating anestimate of a combined image, in accordance with embodiments of theinvention;

FIG. 8 illustrates an exemplary peak sum map of a P image, in accordancewith embodiments of the invention;

FIG. 9 illustrates an exemplary peak sum map of a Q image, in accordancewith embodiments of the invention;

FIG. 10 illustrates an exemplary combined image formed from the imagesof FIGS. 5 and 6, in accordance with embodiments of the invention;

FIG. 11 illustrates an alternate embodiment to the system of FIG. 1showing an alternate way for introducing a converging local oscillatorbeam that passes through the hologram, in accordance with embodiments ofthe invention;

FIG. 12 illustrates an alternate embodiment to the system of FIG. 1showing an alternate way for introducing the local oscillator beam in acollinear storage geometry, in accordance with embodiments of theinvention;

FIG. 13 illustrates an alternate embodiment of the system of FIG. 11showing an alternate way for introducing a local oscillator beam thatmay be modulated by the SLM, in accordance with embodiments of theinvention;

FIG. 14 illustrates an alternate embodiment of the system of FIG. 12showing an alternate way for introducing the local oscillator beam forcollinear holography, in accordance with embodiments of the invention;

FIG. 15 illustrates an exemplary SLM format for use in read operations,in accordance with embodiments of the invention;

FIG. 16 illustrates an alternate embodiment of the system of FIG. 1 forintroducing a local oscillator beam involving two cameras, in accordancewith embodiments of the invention; and

FIG. 17 illustrates an exemplary simulated plot of SNR versus NoisePower for homodyne and non-homodyne detection for systems employingphase shift keying (PSK) and amplifying shift keying (ASK) datarecordation schemes, in accordance with embodiments of the invention.

DETAILED DESCRIPTION

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

DEFINITIONS

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, the term “light source”refers to a source of electromagnetic radiation having a singlewavelength or multiple wavelengths. The light source may be from alaser, one or more light emitting diodes (LEDs), etc.

For the purposes of the present invention, the term “holographicrecording” refers to the act of recording a hologram in a holographicrecording medium. The holographic recording may provide bit-wise storage(i.e., recording of one bit of data), may provide storage of a1-dimensional linear array of data (i.e., a 1×N array, where N is thenumber linear data bits), or may provide 2-dimensional storage of a pageof data.

For the purposes of the present invention, the term “storage medium”refers to any component, material, etc., capable of storing information,such as, for example a holographic storage medium.

For the purposes of the present invention, the term “holographic storagemedium” refers to a component, material, etc., that is capable ofrecording and storing, in three dimensions (i.e., the X, Y and Zdimensions), one or more holograms (e.g., bit-wise, linear array-wise orpage-wise) as one or more patterns of varying refractive index and/orabsorption imprinted into the medium. Examples of holographic mediauseful herein include, but are not limited to, those described in: U.S.Pat. No. 6,103,454 (Dhar et al.), issued Aug. 15, 2000; U.S. Pat. No.6,482,551 (Dhar et al.), issued Nov. 19, 2002; U.S. Pat. No. 6,650,447(Curtis et al.), issued Nov. 18, 2003, U.S. Pat. No. 6,743,552(Setthachayanon et al.), issued Jun. 1, 2004; U.S. Pat. No. 6,765,061(Dhar et al.), Jul. 20, 2004; U.S. Pat. No. 6,780,546 (Trentler et al.),issued Aug. 24, 2004; U.S. Patent Application No. 2003-0206320,published Nov. 6, 2003, (Cole et al.), and U.S. Patent Application No.2004-0027625 (Trentler et al.), published Feb. 12, 2004, the entirecontents and disclosures of which are herein incorporated by reference.

For the purposes of the present invention, the term “data page” or“page” refers to the conventional meaning of data page as used withrespect to holography. For example, a data page may be a page of data,one or more pictures, etc., to be recorded or recorded in a holographicmedium.

For the purposes of the present invention, the term “recording light”refers to a light source used to record information, data, etc., into aholographic recording medium.

For the purposes of the present invention, the term “recording data”refers to storing or writing holographic data in a holographic medium.

For the purposes of the present invention, the term “reading data”refers to retrieving, recovering, or reconstructing holographic datastored in a holographic medium.

For the purposes of the present invention, the term “data modulator”refers to any device that is capable of optically representing data inone or two-dimensions from a signal beam.

For the purposes of the present invention, the term “spatial lightmodulator” (SLM) refers to a data modulator device that is an externallycontrolled, active optical element.

For the purposes of the present invention, the term “refractive indexprofile” refers to a three-dimensional (X, Y, Z) mapping of therefractive index pattern recorded in a holographic recording medium.

For the purposes of the present invention, the term “data beam” refersto a recording beam containing a data signal. As used herein, the term“data modulated beam” refers to a data beam that has been modulated by amodulator such as a spatial light modulator (SLM).

For the purposes of the present invention, the term “coherent lightbeam” refers to a beam of light including waves with a particular (e.g.,constant) phase relationship, such as, for example, a laser beam.

For the purposes of the present invention, the term “reference beam”refers to a beam of light not including data. Exemplary reference beamsinclude non-data bearing laser beams used while recording data to aholographic storage medium. For example, a reference beam may be used tocreate holographic fringes due to interference with a data beam during awrite. Additionally, for example, the term “reference beam” may also bereferred to as a probe beam during, for example, a data read.

For the purpose of the present invention, the term “reconstructed databeam” refers to a beam of light generated by a hologram stored by astorage medium and a reference beam interacting. Exemplary reconstructeddata beams comprise a light beam that is a reconstruction of a data beamused when storing information in a holographic storage medium.

For the purpose of the present invention, the term “collinearholography” refers to a holographic storage technique in which a databeam and a reference beam are aligned on the same axis whenholographically storing information in a storage medium.

For the purposes of the present invention, the term “off-axisholography” refers to a holographic storage technique in which a databeam and a reference beam are not aligned on the same axis whenholographically storing information in a storage medium.

For the purpose of the present invention, the term “phase carrier”refers to a wavefront of an optical beam. Exemplary phase carriers mayinclude a slowly-varying wavefront that would interfere constructivelywith all “+1” pixels of a data beam, and destructively with all “−1”pixels.

For the purpose of the present invention, the term “reserved block”refers to a region of known pixel patterns that are distributedthroughout a data page. Exemplary reserved blocks include an 8×8 patternincluding a pseudo-random pattern with desirable auto-correlationproperties.

For the purpose of the present invention, the term “phase” refers to aposition in the cycle of something that changes cyclically. For example,a sine wave may be expressed as s(t)=A sin(2πft+θ), where A=theamplitude of the wave, f=the frequency of the wave, t=the instantaneoustime, and θ=the phase of the wave. As used herein, exemplary phases, θ,may be 0, 90 degrees, 180 degrees, −90 degrees, etc.

For the purpose of the present invention, the term “phase shift” refersto a constant difference/offset between two instantaneous phases. Forexample, two sine waves may have different phases, where the differencesbetween these phases are referred to as the phase shift. For example, inone example the phase, θ₁, of one wave may be 0 and the phase, θ₂, of asecond wave may be 90. In such, an example, the two waves will bereferred to as having a phase shift of 90 degrees (i.e., θ₂−θ₁=90).

For the purpose of the present invention, the term “phase difference”refers to a difference between the phases of two waves. For example, inone example the phase, θ₁, of one wave may be 0 and the phase, θ₂, of asecond wave may be 90. In such, an example, the two waves will bereferred to as having a phase difference of 90 degrees (i.e., θ₂−θ₁=90).

For the purpose of the present invention, the term “image” refers to atwo-dimensional optical irradiance pattern; or a representation thereofsuch as that captured by a camera.

For the purpose of the present invention, the term “contrast” refers toa difference in brightness between light and dark areas of an image.Additionally, for example, the contrast of an image or pixel capturedduring a data read may be related to the original brightness of therecorded SLM image, and, for example, a negative contrast during a dataread may refer to the image being inverted from the SLM image usedduring the write (e.g., pixels that were on during recordation are offduring the read).

For the purpose of the present invention, the term “image contrastinformation” refers to information regarding a difference in brightnessbetween light and dark areas of an image.

For the purposes of the present invention, the term “beam combiner”refers to a device capable of combining at least two beams of light.Exemplary, beam combiners may include, for example, beam splitters, suchas, for example non-polarizing beam splitters (NPBS), pellicle beamsplitters, etc.

For the purpose of the present invention, the term “camera” refers to adevice capable of capturing an image. Exemplary cameras may include CMOSdetector arrays and charged coupled devices (CCD).

For the purpose of the present invention, the term “processor” refers toa device capable of executing instructions and/or implementing logic.Exemplary processors may include application specific integratedcircuits (ASIC), central processing units and microprocessors, such as,for example, microprocessors commercially available from Intel and AMD.

For the purpose of the present invention, the term “wave plate” refersto a device capable of altering the polarization state of a light wavetraveling through it. Exemplary wave plates include quarter wave plates(QWP) and half wave plates (HWP).

For the purpose of the present invention, the term “local oscillatorbeam” refers to a light beam having a particular frequency and phasedistribution. As used herein, the terms “local oscillator” and “localoscillator beam” may be used interchangeably. In exemplary embodimentsdescribed herein, a local oscillator beam may be coherently mixed orinterfered with a reconstructed data beam.

For the purpose of the present invention, the term “local oscillatorportion” refers to a portion of a light beam or an SLM used to modulatethe local oscillator portion of a collinear beam used for homodyne andor heterodyne detection in a collinear holographic storage system. Forexample, a local oscillator portion of an SLM may be a central portionof the SLM. Similarly, a local oscillator portion of a light beam may bethe portion of the light beam modulated by the local oscillator portionof an SLM.

For the purpose of the present invention, the term “reference pattern”refers to a pattern used by an SLM in a collinear holographic storagesystem for modulating a reference portion of a collinear beam.

For the purpose of the present invention, the term “reference patternportion” refers to a portion of a light beam or an SLM used forgenerating a reference beam in a collinear holographic storage system.For example, a reference pattern portion of an SLM may be an exteriorportion of the SLM. Further, the reference pattern portion may be usedfor forming a reference pattern. Similarly, a reference pattern portionof a light beam may be the portion of the light beam formed by thereference pattern portion of an SLM.

For the purpose of the present invention, the term “optical steeringsystem” refers to one or more components configured to direct a lightbeam in a particular direction. Exemplary optical steering systemscomprise systems configured to direct a combined beam towards a storagemedium so that it passes into the storage medium to generate areconstructed data beam. Exemplary components that may comprise anoptical steering system comprise lenses, mirrors, galvo mirrors, etc.

For the purpose of the present invention, the term “homodyne detection”refers to detecting a reconstructed data beam wherein the reconstructeddata beam is combined with a local oscillator beam in which thefrequency of the local oscillator beam is substantially similar to thefrequency of the reconstructed data beam.

For the purpose of the present invention, the term “heterodynedetection” refers to detecting a reconstructed data beam wherein thereconstructed data beam is combined with a local oscillator beam inwhich the frequency of the local oscillator beam is different than thefrequency of the reconstructed data beam.

For the purpose of the present invention, the term “phase modulationpattern” refers to a two-dimensional distribution of the phase, θ, of anoptical beam, where as noted above the phase, θ, refers to the angularcoordinate of the optical field oscillation cycle.

For the purpose of the present invention, the term “polytopic aperture”refers to device comprising an aperture (e.g. a hole) positioned in theFourier plane. For example, a polytopic aperture may be a hole in asheet of metal positioned in the Fourier plane. During recording, apolytopic aperture may act as a low pass filter of the frequencycomponents of the recording data beam. When reading data, a polytopicaperture may low-pass filter the reconstructed data beam, and filter outreconstructions of overlapping neighbor holograms. A further descriptionof polytopic apertures may be found in Ken Anderson and Kevin Curtis,“Polytopic Multiplexing,” Optics Letters, Vol. 29, No. 12, pp. 1402-1404(Jun. 15, 2004), which is hereby incorporated by reference.

For the purpose of the present invention, the term “filtered intensitylevel” refers to an intensity level for an image. For example, afiltered intensity level may be an intensity level for a pixel of animage. Additionally, a filtered intensity level for a pixel may bedetermined, for example, by subtracting a mean level for all pixels ofthe image from the intensity level for the pixel. Or, for example, afiltered intensity level for a pixel of an image, may be determined byfiltering an image (e.g., by a high pass filter to remove a slowly ornon-varying local oscillator intensity prior to image capture or duringprocessing).

Description

Embodiments of the invention may be used for recovering data inholographic memory systems, such as, for example, data storage andretrieval systems that implement holographic optical techniques such asholographic memory systems.

An embodiment of the system and method of the present invention may usehomodyne detection to amplify and linearize a reconstructed hologram inthe optical domain by adding a known, coherent optical signal (i.e., alocal oscillator beam) to the reconstructed hologram. Amplifying andlinearizing the reconstructed hologram may help to improve the signal tonoise ratio (SNR) for the retrieved data.

Homodyne detection may be accomplished by mixing a page-sized localoscillator with the reconstructed data page signal which is not onlyoptically phase-locked in time, but is everywhere phase-matched in spacesuch that the local oscillator constructively interferences with eachand every data pixel in the hologram simultaneously. This page-sizedlocal oscillator may take the form of a plane wave illuminating theentire detector array. However, alignment tolerances, lens aberrations,wavelength and temperature sensitivities, and a host of other minutedeviations from perfection may introduce small variations in theflatness of the “phase carrier” wavefront bearing the reconstructed datapage. Thus, with prior systems, successfully performing page-widehomodyne detection in such a manner may require expensive, sophisticatedadaptive optic elements and control algorithms in order to phase-matchthe local oscillator to the hologram (or vice-versa). As such,performing homodyne detection in such a manner may not be currentlypractical in a less expensive commercial system, and may only work withprior systems in a carefully controlled laboratory environment, if atall. As used herein the terms “local oscillator” and “local oscillatorbeam” will be used interchangeably.

In order to reduce the need for this specialized and expensiveequipment, embodiments of the present invention may perform homodynedetection without the need for a precise local oscillator. For example,embodiments of the present invention may use two versions of animprecise local oscillator that have a 90 degree phase differencebetween them (i.e., a quadrature relationship). Images of the hologramfor each of these two local oscillator versions may then be captured andprocessed to obtain a high contrast image.

An embodiment of the present invention which performs homodyne detectionusing two versions of a local oscillator is shown in FIG. 1 whichillustrates a simplified diagram of a holographic memory system 100 inaccordance with embodiments of the invention. For purposes ofsimplification, only the components of the holographic memory system 100in the light path between the storage medium 102 and the camera 120 areillustrated. In actual implementation, the holographic memory system 100may include numerous other components, such as, for example, additionallight sources, mirrors, additional beam splitters, etc. For example,holographic memory system 100 may be employed in a holographic memorysystem such as disclosed in U.S. patent application Ser. No. 11/440,370entitled “Illuminative Treatment of Holographic Media” filed May 25,2006, which is hereby incorporated by reference in its entirety.

As illustrated in FIG. 1, holographic memory system 100 may include aholographic storage medium 102, an objective lens 104, a half-wave plate(HWP) 106, a non-polarizing beam splitter (NPBS) 108, another lens 110,a polytopic aperture 112, another lens 114, a polarizing beam splitter(PBS) 116, a spatial light modulator (SLM) 118, a camera 120, and aphase retarder 122. Additionally, holographic memory system 100 of FIG.1 may further include a light source 150, an adjustable HWP 152, asecond PBS 154, a mirror 156, a HWP 157, a galvo mirror 158, and aprocessor 180. The processor 180 may be a processor, such as, forexample a commercially available microprocessor, and the phase retarder122 may be device capable of phase shifting a light beam, such as, forexample, a switchable quarter wave plate (QWP). Additionally, thecombination of lens 110, polytopic aperture 112, and lens 114 may bereferred to as a “4F relay.”

Holographically-stored data may retrieved from the holographic storagemedium 102 by performing a read (or reconstruction) of the stored data.The read operation may be performed by projecting a reference beam 132(also referred to as a probe beam) onto or into the storage medium 102at the same angle, wavelength, phase, position, etc., as the referencebeam used to record the data, or compensated equivalents thereof. Thehologram and the reference beam interact to reconstruct the data beam.As is known to those of skill in the art, the reconstructed data beam134 may comprise the reconstructed data on a phase carrier. Thereconstructed data beam 134 may then pass through lens 104 and HWP 106.Objective lens 104 may be, for example, any type of lens, such as thosecommercially available. Exemplary lenses include, for example, a highnumerical aperture (N.A.) aspheric storage lenses. Lens 104 may also belocated one focal length (i.e., the focal length of lens 104) fromholographic storage medium 102 and may be capable of expandingreconstructed data beam 134. It should be noted that these lenses andtheir locations are exemplary and that in other embodiments, forexample, the lenses may have different locations. For example, in anembodiment it may be desirable for the storage medium 102 to be slightlyout of focus.

The reconstructed data beam 134 may then be combined with a localoscillator beam 136 by NPBS 108. Local oscillator beam 136 may be, forexample, a plane wave. Further, local oscillator beam 136 may begenerated from a portion of the reference beam 132, so that localoscillator beam 136 is temporally coherent with the reconstructed databeam. The local oscillator beam 136 is injected or introduced into thereconstructed object path (i.e., is combined with the reconstructed databeam 134) so that it is collinear with and has the same polarizationstate as the reconstructed data beam 134, although the local oscillatorbeam 136 need not have any special phase relationship to reconstructeddata beam 134. The power of the reflected local oscillator beam 136 maybe set to some power level to effect or cause the desired amount ofoptical gain and dynamic signal range (e.g., 100 times the nominal powerof the reconstructed data beam). This may be accomplished by, forexample, splitting off a portion of the main laser used for generatingthe reference beam 132 by, for example, inserting a HWP 152 in the pathof the main laser beam and adjusting an angle of the HWP 152 to controlthe power of the generated local oscillator beam 136.

FIG. 1 includes a simplified illustration of a mechanism for generatinglocal oscillator beam 136 and reference beam 132, in accordance withembodiments of the invention. As illustrated in FIG. 1, this mechanismmay include a light source 150, an adjustable HWP 152, a PBS 154, mirror156, and galvo mirror 158. It should be noted that this is a simplifieddiagram provided for illustrative purposes and that in actualimplementation additional or alternative components may be used. Forexample, in other embodiments, another 4F relay or another galvo mirrorin tandem with galvo mirror 158 may be used to keep the reference beamfrom walking as the angle changes during reading or writing data.

Light source 150 may be, for example, a laser such as is commonly usedin holographic memory systems. The angle of HWP 152 may be adjusted tomodify the polarization of main beam 160 such that PBS 154 splits of aportion of main beam 156 for local oscillator beam 136. The remainingportion of main beam 160 passes through PBS 154 and may be directed bymirror 156 and galvo mirror 158 to form reference beam 134. By settingthe angle of HWP 152 the power of local oscillator beam 136 may becontrolled. It should be noted that this simplified diagram of theholographic memory system 100 of FIG. 1 is for illustrative purposesonly, and that holographic memory system 100 may include numerous othercomponents, such as additional mirrors, etc. It should also be notedthat FIG. 1 illustrates one example for generating reference beam 132and local oscillator beam 136, and that other implementations may beused, such as, for example, using two separate phase-locked lasers, thegeometry and components of holographic memory system 100 may bedifferent without departing from the invention.

Local oscillator beam 136 may pass through phase retarder 122 prior tobeing injected or introduced into the signal path where local oscillatorbeam 136 may be combined with reconstructed data beam 134. Phaseretarder 122 may be any type of device capable of changing the phase oflocal oscillator beam 136, such as, for example, a commerciallyavailable Ferroelectric Liquid Crystal (FLC) QWP. For example, a QWP(phase retarder 122) may be configured to switch between two stateswhere in one state the fast axis of QWP may be aligned to thepolarization of local oscillator beam 136 in order to impart one phasedelay, but in the other state the slow axis of the may be aligned withthe linear polarization of the local oscillator beam 136, correspondingto an absolute phase difference of 90 degrees. That is, phase retarder122 may be capable of being switched so that phase retarder 122 maychange the phase of the local oscillator beam 136 by 90 degrees.

NPBS 108 combines the local oscillator beam 136 and reconstructed databeam 134 to produce combined beam 138. NPBS 108 may include a partiallyreflective coating such that allows 95% of light to pass through theNPBS 108 and 5% of light to be reflected. In such an example, 95% ofreconstructed data beam 134 will pass through NPBS 108 and 5% will bereflected away. Similarly, 95% of local oscillator beam 136 will passthrough NPBS 108 while 5% of local oscillator beam 136 is reflected andcombined with reconstructed data beam 134. Thus, in this example,combined beam 138 comprises 95% of the reconstructed data beam 134 and5% of the local oscillator beam 136. Further, in this example, theportions of the local oscillator beam 134 (i.e., the portion passingthrough NPBS 108) and reconstructed data beam 136 (i.e., the portionreflected by NPBS 108) not used for generating combined beam 138 may bepassed to a device, such as, for example, a beam block for absorbingthese unused portions of beams 134 and 136.

The combined beam 138 may then pass through lens 110 which focuses thecombined beam 138. Lens 110 may be located, for example, so that itsfront focal plane is the back focal plane of lens 104. The focusedcombined beam 138 may then pass through polytopic aperture 112 which maybe located, for example, 1 focal length from lens 110. Polytopicaperture 112 may be used to filter noise from combined beam 138.Combined beam 138 may then pass through lens 114, which may be located,for example, 1 focal length from polytopic aperture 114. Lens 114 mayexpand combined beam 138 so that beam 138 has a fixed diameter. Combinedbeam 138 may then enter PBS 116 which, because of the polarization ofcombined beam 138, directs combined beam 138 towards camera 120 whichdetects the received image. Camera 120 may be any type of camera capableof detecting combined beam 138, such as, for example, a CMOS detectorarray or charged coupled device (CCD). Although in the embodiment, FIG.1 shows use of NPBS 108 for combining local oscillator beam 136 withreconstructed data beam 134, in other embodiments, other devices may beused, such as, for example, a pellicle beam splitter or a plate beamsplitter.

If the local oscillator beam 136 and the reconstructed data beam 134have the same phase, they will constructively interference. For example,FIG. 2 illustrates exemplary signals where the local oscillator beam 136is coherent and phase-locked in time, and phase matched in space withthe reconstruction data beam 138. SLM pattern signal 202 corresponds toa portion of one row of the SLM pattern used when the hologram isrecorded, where the x-axis is spatial coordinate and the y-axisindicates the optical filed (or electrical field) for SLM pattern signal202 and electric field strength (E_(S)) curve 204 and the y-axisindicates the intensity (or irradiance) for the intensity (I) curve 206.In the illustrated plots, the distance is normalized to SLM pixels.

In the embodiment described in FIG. 1, holographic memory system 100 maystore information using a technique referred to as phase shift keying(PSK) where data may be stored as either “+1” or “−1.” SLM 118 in FIG. 1may be capable of modulating the phase of the light used for recordingthe hologram pixel by pixel. In the embodiment shown in FIG. 1, a “+1”may be represented by a particular a pixel of SLM 118 modulating thedata beam used in recording the hologram so that the data beam at thatparticular pixel location has a particular phase (e.g., 0 degrees). A“−1” may be represented by a pixel of SLM 118 modulating the recordingdata beam so that the data beam is 180 degrees out of phase with the“+1” pixels. In other words, there is a 180 degree phase differencebetween the +1 and −1 pixels. While this technique is exemplified asusing only two phases (referred to as Binary PSK (BPSK), in otherembodiments, other PSK techniques may be used, such as, for example,quaternary PSK (QPSK), i.e., using four phases.

Alternatively, other embodiments may use a technique referred to asamplitude shift keying (ASK), which uses 1's and 0's in recording thedata. Pursuant to this technique, a “1” may be represented by turning aparticular a pixel of SLM 118 on, while a “0” may be represented by aparticular pixel being off.

In FIG. 2, BPSK SLM pattern signal 202 is shown as having sharp edgesand straight lines, with SLM signal pattern signal 202 having anamplitude of +1 at locations where the SLM pixel is one (e.g., betweenlocations −3 and −2, −1 and 0, and +1 and +2, and +2 and +3).Conversely, BPSK SLM signal pattern 202 has a “−1” amplitude value atlocations where the corresponding SLM pixel is zero.

FIG. 2 further illustrates the electric field strength (E_(S)) 204 forthe recovered hologram after being combined with local oscillator beam136 when the hologram is reconstructed. As shown, E_(S) 204 has roundededges due in part to low pass filtering by the polytopic aperture 114during reading and writing of the hologram. Additionally, E_(S) 204exhibits, in this example, a constant +2 offset due to the localoscillator beam 136. FIG. 2 also illustrates the intensity (I) 206 forthe combined signal 138 (i.e., the reconstructed data beam 134 combinedwith the local oscillator beam 136) where the local oscillator beam 136is both phase locked in time, and phase matched in space, with thereconstructed data beam 134, such that the two beams 136 and 134constructively interfere with each and every data pixel in the recoveredhologram simultaneously. This intensity (I) 206 may be calculated asI=|E_(S)+E_(LO)|², where E_(LO) is the electric field strength localoscillator beam 136. As illustrated by the intensity (I) 206 of thecombined beam 138, the local oscillator beam 136 effects or causes aconstant multiple gain to reconstructed data beam 136. In addition tothis constant multiple gain, the local oscillator beam 136 alsointroduces an offset (+2 in this example). A further description of themathematics describing these effects is provided below.

FIG. 3 illustrates a similar diagram to that of FIG. 2, but where thelocal oscillator beam 136 and reconstructed data beam 134 are 180degrees out of phase in time. This results in destructive interferenceand inversion of the detected intensity I. For example, as shown in FIG.3, the SLM signal pattern 302 is identical and the electric fieldstrength E_(S) 304 is identical (but shifted down) relative to that inFIG. 2 due to the local oscillator beam 136 being 180 degrees out ofphase with the reconstructed data beam 138. The intensity (I) 306 issubstantially inverted compared to that of the reconstructed data beam134. For example, as shown in FIG. 3, the intensity (I) 306 is at amaximum when E_(S) 304 is at a minimum, and conversely the intensity (I)306 is at a minimum when E_(S) 304 is at a maximum.

FIG. 4 illustrates a similar diagram to that of FIG. 2, but with thelocal oscillator beam 136 and reconstructed data beam 134 90 degrees outof phase in time. This results in low contrast of the detected intensity(I) 406 of the combined beam 138. For example, as shown in FIG. 4, theSLM signal pattern 402 is identical and the electric field strength,E_(S) 404, for the recovered hologram is identical (but shifted down)relative to that shown in FIG. 2. However, due to the local oscillatorbeam 136 being 90 degrees out of phase with the reconstructed data beam138, the intensity (I) 406 of the combined signal 138 does not exhibit again, but instead the difference between the maximum and minimum of theintensity (I) 406 of the combined signal 138 is reduced. This may resultin blurring of the signal.

Returning to FIG. 2, alignment tolerances, lens aberrations, wavelengthand temperature sensitivities, and a host of other minute deviationsfrom perfection may introduce small variations in the flatness of the“carrier” wavefront for the reconstructed data beam 134. Thus, there maybe a phase difference between the reconstructed data beam 134 (i.e., thehologram) and the local oscillator beam 136. This phase difference mayresult in the image detected at camera 120 containing fringes. Thefringes may delineate regions where the interference between thereconstructed data beam 134 and the local oscillator beam 136 isconstructive, destructive, and/or in-between. The constructiveinterference regions may appear with high contrast; the destructiveinterference regions may also appear with high contrast, but may beinverted; and the in-between regions may appear with low contrast (i.e.,will be blurred).

FIG. 5 illustrates an exemplary image of a hologram detected at camera120. This camera image will be referred to hereafter as the “P” image502. The designation of this image as a P image is an arbitrarydesignation for explanatory purposes. As illustrated, P image 502includes fringes resulting from the phase difference between localoscillator beam 136 and reconstruction data beam 138. As particularlyshown in FIG. 5, constructive and deconstructive interference regions504 are visible as high contrast regions (e.g., the brighter verticalcolumns located at approximately X=0, 125, 250, 375, and 500). The lowcontrast regions 506 are visible in FIG. 5 as dark vertical columns(e.g., at approximately X=75, 175, 325, and 425). (It should be notedthat this image in FIG. 5 is only exemplary and is presented solely forillustrative purposes.)

After capturing the P image, the phase retarder 122 may be switched tocause a 90 degree phase shift in the local oscillator beam 136. Theresulting image, referred to herein for explanatory purposes as the “Q”image, may then be captured by camera 120. Since the phase of the localoscillator beam 136 has changed by 90 degrees, the fringe pattern in thedetected hologram will also shift by 90 degrees. In particular, theregions that were previously high contrast (the constructive anddestructive interference regions) will become low contrast; while thepreviously low contrast in-between regions will become high contrast(constructive or destructive).

FIG. 6 illustrates an exemplary Q image 602 detected at camera 120. LikeP image 502, Q image 602 includes fringes resulting from the phasedifference between local oscillator beam 136 and reconstruction databeam 138. As particularly shown in FIG. 6, constructive anddeconstructive interference regions 604 are visible as high contrastregions (i.e., the brighter vertical columns located at approximatelyX=75, 175, 325, and 425). Conversely, the low contrast regions 606 arevisible as dark vertical columns (i.e., at approximately X=0, 125, 250,375, and 500). Like FIG. 5, it should be noted that this image in FIG. 6is only exemplary and is presented solely for illustrative purposes.

The two quadrature images (i.e., the P and Q images) of FIGS. 5-6 maythen be processed with a digital algorithm executed by a processor 180.In an embodiment, the images include control information consisting of“reserved block” patterns distributed throughout them. A furtherdescription of methods for processing data pixels and reserved blocksaccording to embodiments of the system and method of the presentinvention is disclosed in U.S. application Ser. No. 10/879,847, “Methodand System for Equalizing Holographic Data Pages,” filed on Jun. 28,2004, and in the respective continuation-in-part, U.S. application Ser.No. 11/069,007, “Processing Data Pixels in a Holographic Data StorageSystem,” filed Feb. 28, 2005, and in Mark Ayres, Alan Hoskins, and KevinCurtis, “Image oversampling for page-oriented optical data storage,”Applied Optics, Vol. 45, No. 11, pp. 2459-2464 (Apr. 10, 2006) all ofwhich are hereby incorporated by reference in their entirety.

For example, SLM 118 may be configured (e.g., directed by a processor180) to use a page recording format including reserved blocks, which aresmall regions of known pixel patterns that are distributed throughoutthe data page to be recorded. The reserved blocks contain data-likepseudo-random bit patterns. In the embodiments of FIGS. 5-6, thereserved blocks will described for exemplary purposes as being 8×8 pixelblocks distributed on a grid at a spacing of 64 pixels in both the x andy directions SLM page recording format.

By performing pattern recognition operations (e.g. cross-correlations),the algorithm executed by the processor 180 may be able to determine thecontrast level and inversion state of each reserved block image withinthe two holographic images (e.g., the P and Q images). The reservedblock correlation signal for each image may then be interpolated to theregions between the reserved blocks, serving as an estimate of the phasedifference between the local oscillator beam 136 and the phase carrierfor the reconstructed data beam 134. With this information, thealgorithm may be able to combine the two quadrature images into a singlehigh-contrast image that approximates the image that would have appearedhad a local oscillator beam 136 been phase-aligned everywhere in asingle exposure. The algorithm combines the constructive interferenceregions of the P and Q images with inverted versions of the destructiveinterference regions (thereby restoring polarity). Because of thequadrature relationship between the two images, every pixel appearssomewhere in a high contrast region, or in two medium contrast regionsthat may be combined into an estimate of equivalent quality. A moredetailed description of this exemplary algorithm will be provided belowwith reference to FIG. 7.

FIG. 7 provides an exemplary flow chart of a method for generating anestimate of combined image that might have resulted had the localoscillator beam 136 been phase matched with reconstructed data beam 134.FIG. 7 will also be discussed with reference to the above-discussedFIG. 1. First, the P image may be captured by camera 122 and provided tothe processor 180 (step 702). Further, although this method will bedescribed as being executed by a single processor, in other embodimentsprocessor 180 may comprise multiple separate or interconnectedprocessors without departing from the present invention. After the Pimage is captured, the processor (not shown) may direct phase retarder122 to effect or cause a 90 degree phase shift in the local oscillatorbeam 136 (step 704). The Q image may then captured by camera 122 andprovided to the processor (not shown) (step 706). After both images areprovided to the processor, one of the images may be selected forprocessing (step 708). Although the images are shown in FIG. 7 as beingsequentially processed, it should be understood that, in otherembodiments of this method, the P and Q images may be, for example,processed in parallel. In addition, for the flow diagram of theembodiment shown in FIG. 7, the P image will be processed first.

A region of the image may then select where it is expected that areserved block would be located (step 710). This region, for exemplarypurposes, may be a 20×20 pixel region of the image. As noted above, eachreserved block may be an 8×8 pixel block having a predetermined pattern.This reserved block may, for example, comprise an inner 6×6 pattern andthe 28 pixels surrounding this 6×6 inner pattern may selected such thatcovariance of the 6×6 pattern with any of the 8 edge-bordering pixels iszero. In other words, the inner 6×6 sub-block has half of its pixelstates in common with its eight neighboring sub-blocks, with the otherhalf of its pixel states being different from neighboring sub-blocks. Inthis circumstance, the contribution of cross-correlation noise to thecovariance values neighboring the peak goes to zero. Reserved blocks arefurther described in the above-incorporated U.S. patent application Ser.No. 11/069,007.

The cross-correlation of the expected pattern may be calculated for thisregion (e.g., the 20×20 region of the image, such as Q image 502 and Pimage 602, where it is expected to find this reserved block pattern)(step 712). An embodiment may be practiced with oversampling, such asdescribed in the above-incorporated by reference U.S. patent applicationSer. No. 11/069,007. When practiced with oversampling, the camera pixelsare smaller that the SLM pixels. Thus, the binary 6×6 pattern may becomea multi-level 8×8 pattern on the camera. Additionally, in an embodimentthe mean (D.C.) component (i.e., the offset) of this expected patternmay be removed so that the cross-correlation with a random camera blockwill tend towards zero rather than the product of the means.Accordingly, in such an example the expected pattern may be an 8×8pattern representing the 6×6 inner pattern used in recording thehologram as it would appear on the camera pixels (which may be smallerthan the SLM pixels), minus the average component. Thus, this expectedpattern (e.g., the 8×8 pattern) may be referred to as an oversampledD.C.-free version of the known pixel pattern (e.g., the 6×6 pattern) ofthe reserved block expected to be located in the region.

These cross-correlations may be determined by calculating thecorrelation for the 8×8 expected pattern at one location in the region(e.g. the 20×20 region) by, for example, summing the combined values ofthe expected pattern (e.g., the 8×8 pattern) with the values of thecorresponding pixel in the region. Then, the expected pattern may beshifted by one pixel in the region and the combined values of theexpected pattern and region's pixels summed, and so on, until thecorrelation value for every possible location in the region for theexpected pattern is calculated. For example, with a 8×8 expected patternand a 20×20 region, this will result in 13×13 or 169 separatecorrelation values being calculated for the region.

This calculated cross-correlation will have a peak value (eitherpositive or negative) that may be located at the location of thereserved block if the reserved block is located in a high contrastregion. The intensity and location of this peak may be stored by theprocessor. That is, in selecting the pixel location with the peak value,this exemplary algorithm uses the absolute value of thecross-correlation and selects the pixel location with the largestabsolute value of the cross-correlation.

Because it is possible that the location of the reserved block might notfall directly on a particular pixel, a peak sum may also be used for thecross-correlation. For example, due to various alignment errors, thereserved block might be shifted by a fraction of a pixel at camera 120.To help compensate for this, a peak sum for the cross-correlation may becalculated by summing the cross-correlation values for the pixellocation where the maximum absolute value of the cross-correlation isfound along with a number of pixel locations surrounding this pixellocation. For example, the peak sum may be calculated by summingtogether the cross-correlations for a neighborhood of 2×2 pixellocations including the pixel location with the cross-correlation havingthe maximum absolute value. Or, in other examples, the 2×3, 3×2 or 3×3pixels surrounding the pixel location with the largest absolute valuemay be used in calculating the peak sum.

After determining the cross-correlation in step 712, the next step is todetermine whether there are other reserved blocks for which across-correlation value should be determined (step 714). For exemplarypurposes, there are 11×12 equally spaced reserved patterns located inthe SLM page format of FIGS. 5 and 6. If there are more reserved blocksfor which there may be a need to calculate the cross-correlation, themethod may be repeated by returning, as shown in FIG. 7, to step 710with the cross-correlation for the next reserved block being calculated(steps 712). In the next step, it may be determined whether thecross-correlation values for the other image are to be calculated (step716). If so, the method may again be repeated by returning, as shown inFIG. 7, to step 708 with the cross-correlations being determined foreach reserved block of this image (e.g., the Q image) being calculated(steps 710-714).

These calculated cross-correlations values may be used to generate peaksum maps for the Q and P images that indicate the maximum calculatedcross-correlation peak sum value for each of the reserved blocks foreach respective figure. These peak sum maps will have maximum intensityvalues (i.e., values approaching the absolute value of the product ofthe norm of the 8×8 expected pattern and the norm of the 8×8 camerapattern at that location) for reserved block located in high contrastregions and minimum intensity values (i.e., values near 0) for reservedblocks located in low contrast regions. These peak sum maps may serve asmeasurements of the relative phase difference between the localoscillator beam 136 and the reconstructed data beam 134.

FIG. 8 illustrates an exemplary peak sum map 800 of the calculated peakintensity value 802 for each of the 11×12 reserved blocks of the SLMpage format for the P image illustrated in FIG. 5. As shown in FIG. 8,the calculated peak intensity values have maximum intensity values(i.e., values approaching plus or minus the maximum value) at locationscorresponding to the high contrast regions 504 of the P imageillustrated in FIG. 5. FIG. 9 illustrates a similar exemplary peak summap 900 of the calculated peak intensity value 902 for each of the 11×12reserved blocks of the SLM page format for the Q image illustrated inFIG. 6. As shown in FIG. 9, the calculated peak intensity values havemaximum intensity values (i.e., values approaching plus or minus themaximum value) at locations corresponding to the high contrast regions604 of the P image illustrated in FIG. 5.

Returning to the flow chart of FIG. 7, after the peak sums for thereserved blocks of each image are calculated (step 712), the peak sumvalues may be interpolated for each image (i.e., the Q and P images) toall pixel locations for the respective image (step 718). Thisinterpolation may be accomplished, for example, by performing a simplelinear two-dimensional (2D) interpolation. In the next step, thecombined image may be calculated (step 720). This combined image may becalculated by multiplying the interpolated peak sum value for each pixeland the image value for each pixel of a respective image and then addingit to the value of each pixel for the other image multiplied by theinterpolated peak sum value for that pixel. That is, the intensity foreach pixel of the combined image may be calculated using the followingformula:

Combined Pixel Intensity=[(P image pixel intensity)*(interpolated peaksum for P image pixel)]+[(Q image pixel intensity)*(interpolated peaksum for Q image pixel)]

In an alternative embodiment, the combined image may be calculated bymultiplying each pixel of the respective image by a scaling factordetermined by a formula involving both interpolated peak sum values forthat pixel. For example, the combined pixel intensity may be determinedin a manner that normalizes the expected intensity and minimizes theexpected noise according to the formula

$I_{comb} = {{I_{P}\frac{P}{{P}^{2} + {Q}^{2}}} + {I_{Q}\frac{Q}{{P}^{2} + {Q}^{2}}}}$

where I_(comb) is the combined pixel intensity, I_(P) is the P imagepixel intensity, I_(Q) is the Q image pixel intensity, P is theinterpolated peak sum for the P image, and Q is the interpolated peaksum for the Q image.

In an alternative enhancement to this procedure, the interpolated peaksum values for a given pixel may be compared to see if the absolutevalue of one dominates. For example, if |P|>v|Q| where v is a thresholdvalue for example equal to three, then the interpolated peak sum valueof Q for that pixel may be replaced by zero so that the P image pixelvalue will entirely determine the combined pixel intensity value.Similarly, if |Q|>v|P| the interpolated peak sum value of P for thatpixel may be replaced by zero. This enhancement may reduce the risk ofselecting a false peak due to small interpolated peak sum valuescorresponding to low contrast regions where the true cross correlationpeak is weak.

In yet another enhancement to this procedure, an estimate of the localoscillator intensity may be subtracted from the P and Q pixelintensities prior to use in determining the combined pixel intensity.This may be accomplished by subtracting the average value of all the Ppixel intensities from each pixel of the P image, and likewisesubtracting the average value of all the Q pixel intensities from eachpixel of the Q image thereby producing a filtered intensity level foreach pixel in the P image, and a filtered intensity level for each pixelin the Q image. Alternatively, the averaging could be done over smallersub-blocks, e.g., 32×32 pixels; or the P and Q images could be filteredwith a high-pass filter to remove the slowly-varying (and ideallynon-varying) local oscillator intensity.

The combined image represents the image that would have been detectedwith a single, phase matched local oscillator. FIG. 10 illustrates anexemplary combined image of the images of FIGS. 5 and 6. As shown inFIG. 10, the high contrast regions have been combined so that the entirecombined image is a high contrast image. This combined image may then befurther processed using conventional detection methods. In addition,information from the P and Q reserved block correlation operations maybe incorporated into this processing. For example, the correlationscalculated for the P and Q images may be used in calculating thecorrelations rather than recalculating the correlations using methodsdisclosed in the above-incorporated by reference U.S. application Ser.No. 10/879,847 and U.S. application Ser. No. 11/069,007.

The presently described embodiment (e.g., system 100, as well as systems1100, 1200, 1300, 1400, and 1600 to be described hereafter) may providethe ability to detect the sign of the optical field (as well as themagnitude), which permits the data to be encoded with phase shift keying(PSK) rather than the typically used amplitude shift keying (ASK). PSK,however, may not often be used for holographic storage since aconventional photodetector may not detect the difference between the twophases. However, all other factors being equal, the use of PSKmodulation may produce an immediate improvement of 3 dB in signal tonoise ratio when compared to ASK. Furthermore, using PSK for dataencoding provides additional benefits for holographic storage. Inparticular, the “D.C. hot spot” that may appear at the center of theFourier plane of an ASK-modulated data page may be eliminated (i.e., nophase mask may be required for Fourier plane recording), and thecontribution of certain holographic noise terms (e.g., intraobjectgratings that are written between individual pairs of pixels in theobject beam) may be greatly reduced. The general improvement inuniformity of the object beam may also enhance the capability of theholographic storage medium to record holograms without local sensitivitydepletion, or effects from uneven shrinkage and uneven bulk indexchanges.

An additional benefit of homodyne detection according to embodiments ofthe present invention is that the optical gain introduced by thehomodyne detection makes the total exposure time for the two quadratureimages potentially far less than the exposure time required for a singlenon-homodyne image. Additionally, in order to minimize the number offringes in the P and Q images, embodiments of the present invention mayinclude components for ensuring that the holographic storage medium 102maintains an accurate position. For example, the holographic memorysystem may include components for moving the holographic storage mediumin x, y, and z directions (i.e., back and forth, up and down, and rightto left). Embodiments of the system of the present invention may furtherinclude processing for determining errors in the positioning of theholographic storage medium 102 by determining the x component, ycomponent, and quadratic component of the fringe pattern, and adjustingthe storage medium's 102 position accordingly. For example, in thecomplex wavefront of the difference between the reconstructed data beamphase carrier and the local oscillator as measured by the P and Qinterpolated peak sum images and having the form exp(i*arctan 2(Q, P));the x positioning error will be manifested as a factor of the formexp(i*f_(x)*x), the y positioning error will be manifested as a factorof the form exp(i*f_(y)*y), and the z positioning error will bemanifested as a factor of the form exp(i*f_(z)*(x²+y²)/2), where f_(x)is a constant proportional to the x positioning error, f_(y) is aconstant proportional to the y positioning error, and f_(z) is aconstant proportional to the z positioning error.

FIG. 11 illustrates an alternate embodiment of the system of FIG. 1where the local oscillator beam may be introduced as a converging beamthat passes through the hologram. As shown in FIG. 11, holographicmemory system 1100 may include a holographic storage medium 1102, a lens1104, a half-wave plate (HWP) 1106, another lens 1110, a polytopicaperture 1112, another lens 1114, a polarizing beam splitter (PBS) 1116,a spatial light modulator (SLM) 1118, a camera 1120, a phase retarder1122, and another lens 1124. These components may be the same or similarto those discussed above with reference to system 100 of FIG. 1. (Forsimplification, only the components of the holographic memory system1100 in the light path between the storage medium 1102 and the camera1120 are illustrated in FIG. 11.) Further, holographic memory system1100 may function identically or similarly to that of the system of FIG.1 with, for example, the exception that the local oscillator beam 1136may be combined with the reconstructed data beam 1134 contact within theholographic storage medium 1102. System 1100 may have the advantage ofnot incurring a power loss for the reconstructed data beam 1134 or thelocal oscillator beam 1136, but may also introduce undesirableaberrations or scattering of the local oscillator beam 1136. Further, aswith the embodiment of FIG. 1, phase retarder 1122 may be a QWP, suchas, for example, a switchable FLC QWP.

In the holographic memory system 1100 of FIG. 11, local oscillator beam1136 passes through phase retarder 1122 (e.g., a QWP) where its phasemay be shifted by 90 degrees such as in system 100 of FIG. 1. Localoscillator beam 1136 then passes through lens 1124 which may be locatedso that its back focal plane is coincident with the front focal plane oflens 1104. Lens 1124 may focus local oscillator beam 1136 so that itpasses through the same location of holographic medium 1102 thatreference beam 1132 passes through. Local oscillator beam 1136 may thencombine with the reconstructed data beam 1134 generated by referencebeam 1132 passing through holographic storage medium 1102. The combinedbeam 1138 then passes through lens 1104, HWP 1106, lens 1110, polytopicaperture 1112 and lens 1114, all of which may function the same orsimilar to the corresponding components of system 100 of FIG. 1. PBS1116 may then reflect combined beam 1138 towards camera 1120, whichcaptures an image of combined beam 1138. Camera 1120 and PBS 1116 may bethe same type of components as discussed with reference to of system 100of FIG. 1. Further, in system 1100, as with system 100 of FIG. 1, localoscillator beam 1136 may be generated by splitting off a portion of amain beam used in generating reference beam 1132.

Like system 100 of FIG. 1, phase retarder 1122 may be used to shift thephase of local oscillator beam 1136 by 90 degrees in order to generate aP image (e.g., un-shifted phase of local oscillator beam 1136) and a Qimage (e.g., local oscillator beam 1136 shifted by 90 degrees). Theresulting images may then be processed as discussed above with referenceFIGS. 1 and 7 to generate a combined image.

In an alternative embodiment, quadrature homodyne detection may bepracticed using other holographic storage architectures or geometries,such as that illustrated in FIG. 12. FIG. 12 illustrates one possiblesystem, referred to generally as 1200, for introducing a localoscillator beam in a collinear storage geometry. As shown, FIG. 12includes holographic storage medium 1202, a lens 1204, QWP 1206, a NPBS1208, another lens 1210, polytopic aperture 1212, another lens 1214, aPBS 1216, a SLM 1218, a camera 1220, and another QWP 1222. Thesecomponents may be the same or similar to those components as discussedabove with reference to system 100 of FIG. 1. As one of skill in the artwould recognize, holographic memory system 1200 exemplified in FIG. 12is for illustrative purposes and actual implementation of system 1200may include numerous additional components without departing from thespirit and scope of the present invention.

In FIG. 12, reference beam 1232 may enter PBS 1216, which, due to thepolarization of reference beam 1232, reflects reference beam 1232towards SLM 1218. As with other collinear storage geometries, SLM 1218may be set so that the SLM portion of SLM 1218 reflects reference beam1232 during read operations. Reference beam 1232 may then pass throughlens 1214, polytopic aperture 1212, lens 1210, NPBS 1208, QWP 1206 andlens 1204. Lens 1204 may then focus reference beam 1232 into or ontoholographic storage medium 1202, which may have a reflective coatinglocated on its backside, thus generating reconstruction data beam 1234which then passes back through the components in the opposite directiontowards PBS 11120.

A local oscillator beam 1236 may be combined with reconstruction databeam 1234 by, for example, NPBS 1208. Like system 100 of FIG. 1 orsystem 1100 of FIG. 11 described above, local oscillator beam 1236 maybe generated by splitting off a portion of a main beam used ingenerating reference beam 1232. As shown in FIG. 12, local oscillatorbeam 1236 passes through QWP 1222 (which functions as a phase retarder)prior to being combined with reconstruction data beam 1234 by NBPS 1208.Like system 100 of FIG. 1 or system 1100 of FIG. 11 described above, QWP1222 may be a FLC QWP that may be switched by a processor (not shown) sothat it either allows local oscillator beam 1236 to have its phase leftun-shifted or shifted by 90 degrees.

Combined beam 1238 may then pass through lens 1210, polytopic aperture1212, and lens 1214. Combined beam 1238 may then be reflected by PBS1216 towards camera 1220, which captures an image of combined beam 1238.In this manner, camera 1220 may capture a P image (e.g., by QWP 1222 notretarding the phase of local oscillator beam 1236) and a Q image (e.g.,by QWP 1222 retarding the phase of local oscillator beam 1236 by 90degrees). The captured P and Q images may then be processed to generatea combined high contrast image, in the same manner as discussed abovewith reference to FIGS. 1 and 7.

FIG. 13 illustrates an alternate embodiment of system 100 of FIG. 1where the local oscillator beam may be introduced through the SLM. Asshown in FIG. 13, holographic memory system 1300 includes a holographicstorage medium 1302, a lens 1304, a QWP 1306, another lens 1310, apolytopic aperture 1312, another lens 1314, another QWP 1322, a PBS1316, an SLM 1318, and a camera 1320. These components may be the sameor similar to those discussed above with reference to system 100 ofFIG. 1. Further, for example, polytopic aperture 1312 may be apartially-silvered polytopic aperture. In holographic memory system1300, the wavefront of the local oscillator beam 1336 may be adaptivelymodulated by SLM 1318 (i.e., by the same SLM that is used during a datawrite) to match the phase carrier of the reconstructed data beam 1334.

The following provides an exemplary description of holographic memorysystem 1300 for adaptively modulating the local oscillator beamwavefront: During a read operation, s-polarized light (i.e., linearpolarization perpendicular to the plane of FIG. 13) in a localoscillator beam 1336 may reflect off the interface in the PBS 1316 andilluminate SLM 1318. SLM 1318 may impart local oscillator beam 1336 witha desired phase modulation pattern, and may then reflect the localoscillator beam 1336 such that beam 1336 has p-polarization. Localoscillator beam 1336 may propagate through PBS 1316 and through QWP1322, which changes the polarization of local oscillator beam 1336 tocircular. Local oscillator beam 1336 may then be focused by lens 1314downwardly towards partially-silvered polytopic aperture 1312, which maybe located in a Fourier plane with respect to SLM 1318. A portion (e.g.,about 5%) of local oscillator beam 1336 may be reflected bypartially-silvered polytopic aperture 1312, thereby changing thehandedness of the circular polarization. The light may then propagateback through QWP 1322, which changes the polarization of the reflectedlight to linear s-polarized. Local oscillator beam 1336 may then reflectoff of the interface within PBS 1320 towards camera 1320, which may beat an image plane with respect to SLM 1318. Thus, the local oscillatorbeam wavefront appearing on SLM 1318 may be recreated at the camera 1320(though inverted about the x and y axes).

Concurrently with the local oscillator beam 1336 generation, a referencebeam 1332 illuminates holographic storage medium 1302, for example, witha linear s-polarization. The hologram in medium 1302 diffracts lightinto reconstructed data beam 1334 which may then pass through lens 1304,QWP 1306, and lens 1310 (collectively referred to as a “4F lens relay”).Within this path, QWP 1306 changes the polarization of reconstructeddata beam 1334 from linear s to circular, having the same handedness asthe local oscillator beam 1336 after reflection off of the partiallysilvered polytopic aperture 1312. It should be noted that the choice ofleft or right handedness within the path between QWP 1306 and QWP 1322is arbitrary. However, the co-propagating signal and local oscillatorbeams may have the same handedness. Upon striking partially silveredpolytopic aperture 1312, some portion of the reconstructed data beam1334 (for example, about 95%) may be transmitted through and mixedcoherently with the local oscillator beam 1336 to form a combined beam1338. From there, the combined beam 1338 may pass through QWP 1322,which changes the polarization back to linear s-polarization. Thecombined beam 1338 beam may subsequently reflect off of the PBS 1316interface towards camera 1320, where it forms an image of the datapattern in superposition with the coherent local oscillator.

Data recovery operations in system 1300 may then be performed using SLM1318 to adaptively create a local oscillator wavefront in phase with thesignal beam phase carrier everywhere across the image, using thealgorithms described herein to measure the phase carrier of the signalso that it may be duplicated by the local oscillator beam 1336. Forexample, at the start of a series of data read operations, SLM 1318 (atthe direction of a processor (not shown)) may create a constant phaselocal oscillator beam 1336 (i.e., the phase of the local oscillator beam1336 may be the same across its wavefront) that is used to recover a Pimage of the first hologram by camera 1320 that may then provided to aprocessor (not shown). A constant local oscillator with a 90 degreesphase difference from the phase of the local oscillator beam 1336 ingenerating the P image may then be used to recover a Q image by camera1320 of the same hologram from medium 1302. A map of the signal beamphase carrier may then be determined by processor 1340 using therecovered P and Q images.

For example, the P and Q images may be processed such as described withreference to FIG. 7 to determine the contrast level for each pixel inthe recovered P and Q images. Using the determined contrast level foreach pixel, a corresponding phase error for each pixel may bedetermined. For example, if a pixel is determined to have a normalizedcontrast level of +1 for a particular pixel of the P image, the phaseerror may be determined to be 0. Similarly, if a normalized contrastlevel of +1 is determined for a particular pixel of the Q image, thephase error may be determined to be 90 degrees. Likewise if a particularpixel is determined to have a normalized contrast level of −1 for the Pimage, then the phase error may be determined to be 180 degrees.Further, if a normalized contrast level of ½ (i.e., 0.5) is determinedfor a pixel of the P image and a normalized contrast level of ½ (i.e.,0.5) is determined for the same pixel of the Q image, a phase error of45 degrees may be determined. For example, this phase error may becalculated where the phase error=arctan 2(Q,P), where arctan 2 is thefour quadrant inverse tangent function.

The processor (not shown) may then use the determined phase error map todirect SLM 1318 of system 1300 to effect or cause respective phaseshifts by each corresponding pixel of SLM 1318 to generate a wavefrontof local oscillator beam 1336 that will be phase matched in time andspace with the phase carrier of reconstructed data beam 1334. Thus, ahigh contrast image of the hologram may be recovered with a single imageexposure by SLM 1318. The processor (not shown) may then repeatedly usethe same wavefront to recover a sequence of holograms since therecording and recovery conditions of neighboring holograms will besimilar, and their phase carriers will be substantially identical.

Additionally, in another embodiment of system 1300, the processor (notshown) further uses an adaptive algorithm to make small changes in thelocal oscillator wavefront by adjusting the phases of the pixels of SLM1318 in order to track the changes in the phase carriers of subsequentholograms that occur as the conditions of recording and recovery divergefrom the conditions when the original phase carrier was measured. As anexample of such an algorithm, the processor (not shown) may deliberatelyintroduce a small alternating constant phase offset in the wavefront oflocal oscillator beam 1. For example, the processor (not shown) may usea +15° phase advance for the whole local oscillator pattern for a firsthologram recovery, and a −15° phase retardation for the next hologramrecovery. Such small phase perturbations may only very slightly degradethe SNR of the two images, and thus they may still be recovered.However, these small phase perturbations would introduce a slightreserved block cross-correlation peak sum intensity modulationindicating the gradient of the absolute phase difference between thelocal oscillator beam 1336 and the reconstructed data beam's 1334 phasecarrier. Where the cross correlation peak sums from the phase advancedand retarded images have equal intensity, the phase difference will bezero. However, where the phase advanced and retarded imagescross-correlation peak sums have unequal intensity, the local oscillatorbeam 1336 has a phase error which may be reduced towards zero byadjusting the phase in the direction (advancement or retardation) thatgave the stronger cross correlation peaks. By means of such a “wobbleservo,” the processor (not shown) may recover long sequences ofholograms with only a single exposure apiece, even while the phasecarriers of the individual holograms were changing over the sequence. Inan alternative embodiment, the optical path that images SLM 1318 ontocamera 1320 could be different. For example, instead of a partiallysilvered surface within polytopic aperture 1312, a partially-silveredsurface could be placed between QWP 1306 and lens 1310 in the commonfocal plane of lens 1304 and lens 1310. This surface would also reflecta portion of local oscillator beam 1336 to form an image of SLM 1318upon camera 1320. As such, in this example, the partially silvered QWPwould function as a beam combiner for combining the local oscillatorbeam 1336 and the reconstructed data beam 1334 to form combined beam1338.

FIG. 14 illustrates alternate embodiment of the system of FIG. 12showing an alternate way for introducing the local oscillator beam forcollinear holography. System 1400 of FIG. 14 differs from the geometriesof prior systems 100, 1100, 1300 and especially 1200 in that the SLMitself is used to generate both the reference and local oscillatorbeams. The reference pattern comes from a region of the SLM reserved forthe reference beam as practiced in other collinear recording, and thelocal oscillator beam comes from an area of the SLM that is used for thedata pattern during recording.

As shown in FIG. 14, holographic storage medium 1400 includes aholographic storage medium 1402, a lens 1404, a QWP 1406, a PBS 1408, anSLM 1410, and a camera 1412. These components may be, for example, thesame type of components discussed above with reference to FIG. 1. Itshould be noted that holographic memory system 1400 is for illustrativepurposes, and actual implementation of system 1400 may compriseadditional components.

In a read operation using system 1400, a reference beam 1432 may bedirected towards PBS 1408. Reference beam 1432 may be a coherent lightbeam and of the same or similar type as the reference beam used inwriting data to holographic storage medium 1402. Reference beam 1432 mayalso be referred to as an SLM illumination beam. PBS 1408 reflectsreference beam 1432 towards SLM 1410, which may be used to generate aparticular pattern. FIG. 15 illustrates an exemplary format for SLM 1410for use in read operations. As shown in FIG. 15, SLM pattern 1500 mayinclude a central circular local oscillator portion 1502 and an outsidereference pattern portion 1504. In other collinear read operations, thelocal oscillator portion 1502 may be set so that all pixels in thisportion are off. In the embodiment shown in FIG. 15, however, localoscillator portion 1502 may be set so that portion 1502 reflects aportion of reference beam 1432 that functions as a local oscillatorbeam.

A further description of how the pixels of local oscillator portion 1502may be set is provided below. Reference pattern portion 1504 may be setso that its pixels are set in the same manner used during writing thehologram. In one example, in order to generate the Q and P images,holographic memory system 1400 may use a switchable phase element, suchas an QWP (not shown) in the SLM illumination path that generates thetwo desired phase profiles (e.g., the local oscillator region is changedby 90 degrees while the reference region remains unchanged) and imagesthem onto the SLM. This example may be used such that a Q image may becaptured where the QWP (not shown) is set such that it leaves the localoscillator portion 1504 of the reference beam 1432 unchanged and a Pimage may be captured by setting the QWP (not shown) such that itchanges the local oscillator portion 1504 of the reference beam 1432 by90 degrees and leaves the reference region portion 1502 unchanged. The Qand P images may then be processed, such as described above withreference to FIG. 7 to generate a combined image.

In another example, SLM 1410 of FIG. 14 may be an SLM capable ofproducing the 90 degree phase shift in the local oscillator portion 1502(e.g., a phase-modulating SLM with better than binary phase resolution).This system may function similar to the above-described embodiment ofFIG. 13. For example, an initial P image may be captured by camera 1412where all pixels in the local oscillator portion 1502 are set to modifythe reference beam 1432 by a particular phase. The phase shift of thelocal oscillator portion 1502 may then be shifted by 90 degrees and a Qimage captured. These captured P and Q images may then be processed suchas described above with reference to FIG. 13 to determine acorresponding phase error for each pixel. Using this determined phaseerror information, a phase shift for each pixel in the local oscillatorportion 1502 of SLM 1410 may then be determined. For example, for pixelsin the local oscillator portion 1502 that are determined to be highcontrast may be set such that these pixels cause the same phase shift asduring the initial image capture, while those pixels with low contrastare set such that those pixels cause a 90 degree phase shift from thephase shift during the initial capture. That is, the phase shift foreach pixel in the local oscillator portion 1504 may be set in accordancewith the determined phase error for the respective pixel so that allpixels will result in a high contrast image when captured by camera1412.

By individually setting the phase shift for each pixel of localoscillator portion 1404, it may not be necessary to generate the P and Qimages, and instead one initial setting may be determined that isperiodically checked and adjusted to ensure a high contrast image isbeing captured by camera 1412. Homodyne recovery may then be performedwith a single camera exposure without the need for the quadraturerecombination procedure. Alternatively, homodyne recovery may then beperformed with a single camera exposure by means of a wobble servo aspreviously described.

Additionally, in the embodiment of FIG. 15, it may also be desirable tomodulate the region-wise amplitude as well as the phase for each pixel.For example, the processor (not shown) in processing the image maydetermine relative intensity levels for each pixel of the high contrastimage. This determined relative intensity level information may then beused to boost the intensity of pixels with low intensities and/or retardthe intensity of those pixels with high intensities. For example, if apixel that is “on” is found to have a +1 intensity that is higher thanthe intensity of another +1 pixel, the intensities of these pixels maybe adjusted by the processor (not shown) to direct the SLM to effect acorresponding increase or decrease in the intensity of the correspondingpixel. This may be accomplished for example, using separate, cascadedphase and amplitude SLMs. Adjusting the relative intensity levels ofeach pixel may aid in obtaining an image with a more homogenousintensity level and help increase the SNR for the recovered data.

FIG. 16 illustrates an alternate embodiment of the system of FIG. 1 forintroducing a local oscillator beam involving two cameras, which isgenerally referred to as holographic memory system 1600. System 1600 isidentical to system 100 of FIG. 1 with the exception that in system, twocombined beams 1638 and 1639 are generated. For simplification, only thecomponents of the holographic memory system 1600 in the light pathbetween the storage medium 1602 and the camera 1620 are shown in FIG.16. In actual implementation, the holographic memory system 1600 mayinclude numerous other components, such as, for example, a light source,mirrors, additional beam splitters, etc.

As shown in FIG. 16, holographic memory system 1600 includes aholographic storage medium 1602, a lens 1604, a HWP 1606, a NPBS 1608,another lens 1610, a polytopic aperture 1612, another lens 1614, a PBS1616, a SLM 1618, a camera 1620, as well as a QWP 1622, another lens1640, a second polytopic aperture 1642, another lens 1644, and a secondcamera 1646. In system 1600, the local oscillator beam 1636 andreconstructed data beam 1634 (generated by passing reference beam 1632through medium 6102) may be combined by NPBS 1608 to form a combinedbeam 1638, as in system 100 of FIG. 1, with combined beam 1638 being beprocessed in the same or similar manner as discussed above withreference to system 100 of FIG. 1. Further, in system 1600, NPBS 1608may include a partially reflective coating that reflects about 50% ofincident light while allowing the remaining about 50% incident light topass through NPBS 1608.

In system 100 of FIG. 1, the portion of the reconstructed data beam 134reflected by NPBS 108 is not used. In system 1600 of FIG. 16, however,this portion of reconstructed data beam 1634 reflected by NPBS 1608 maybe combined with the portion of local oscillator beam 1636 passingthrough NPBS 1608 to form a second combined beam 1639. This secondcombined beam 1639 may then pass through lens 1640, a second polytopicaperture 1642, and another lens 1644. These components may be the sameor similar to the components used in the path of combined beam 1638. Animage of the second combined beam 1639 may then be captured by secondcamera 1646.

In system 1600 of FIG. 16, two separate Q images may be captured withone captured by camera 1620 and the second by camera 1646. Likewise, twoseparate P images may be captured. These Q and P images may be processedto produce two separate combined images using methods such as discussedabove with reference to FIG. 7. These two combined images may then becombined to produce an even higher quality image, such as, for example,by taking the difference of the two images after resampling.

It should be noted that the systems 100, 1100, 1200, 1300, 1400 and 1600described above are exemplary only and that exemplary systems may bemodified or other systems used without departing from spirit and scopeof the present invention. For example, it may not necessary to use arelay lens (e.g., lenses 110 and 114) and/or a polytopic filter.

The following provides an explanation of some of the mathematicsillustrating some of the benefits of using homodyne detection such asdescribed above. For conventional non-homodyne detection, the irradiancepattern may be represented as impinging upon the detector as:

$\begin{matrix}\begin{matrix}{{I_{cam}\left( {x,y} \right)} = {{{E_{S}\left( {x,y} \right)} + {E_{N}\left( {x,y} \right)}}}^{2}} \\{{= {{E_{S}}^{2} + {E_{N}}^{2} + {2{E_{S}}{E_{N}}\cos \; \varphi_{S - N}}}},}\end{matrix} & (1)\end{matrix}$

where E_(S)(x,y) and E_(N)(x,y) are the scalar complex amplitudes of theholographic signal and the coherent optical noise, respectively. Therelative phase difference between the two fields, φ_(S-N), is a randomvariable, so the cos factor in the final term swings between +1 and −1randomly. This term, which has the signal multiplied by the noise ratherthan adding to it, is a major limiting noise factor in the practicaldevelopment of holographic data storage.

For homodyne detection, the expression may become:

$\begin{matrix}\begin{matrix}{I_{homo} = {{E_{LO} + E_{S} + E_{N}}}^{2}} \\{= {{E_{LO}}^{2} + {E_{S}}^{2} + {E_{N}}^{2} +}} \\{{{2{E_{LO}}{E_{S}}\cos \; \varphi_{{LO} - S}} +}} \\{{{2{E_{LO}}{E_{N}}\cos \; \varphi_{{LO} - N}} +}} \\{{{2{E_{S}}{E_{N}}\cos \; \varphi_{S - N}},}}\end{matrix} & (2)\end{matrix}$

where E_(LO) is the complex amplitude of the local oscillator. Since wewill use a strong local oscillator to produce a lot of optical gain, wemay safely postulate |E_(LO)|>>|E_(S)| and |E_(LO)|>>|E_(N)|, so thatterms not involving E_(LO) become negligible. Thus,

$\begin{matrix}\begin{matrix}{{I_{homo}\left( {x,y} \right)} \cong {{E_{LO}}^{2} + {2{E_{LO}}{E_{S}}\cos \; \varphi_{{LO} - S}} +}} \\{{2{E_{LO}}{E_{N}}\cos \; \varphi_{{LO} - N}}} \\{= {{E_{LO}}^{2} + {2{{{E_{LO}}\left\lbrack {{\pm {E_{S}}} + {{E_{N}}\cos \; \varphi_{{LO} - N}}} \right\rbrack}.}}}}\end{matrix} & (3)\end{matrix}$

In equation (3), cos φ_(LO-S)=±1 is used since the quadraturerecombination process has forced cos φ_(LO-S) to match the data polaritythroughout the image. Thus, the detected signal may comprise again-enhanced signal and additive noise, and a constant background term.As shown in equation (3), the troublesome multiplicative noise term hasvanished, and the signal term may be proportional to the optical fieldrather than the irradiance.

The improvement in performance may be quantified by simulating theeffects of noise on holograms detected with and without homodynedetection. FIG. 17 illustrates an exemplary simulated plot 1700 of theSNR versus Noise Power for homodyne and non-homodyne detection forsystems employing the PSK and ASK data recordation schemes. It should benoted that this simulated plot is provided to illustrate some of thepotential benefits of the embodiments of the present invention. Thehorizontal axis indicates the amount of coherent noise added to thenominally noise-free holographic image, and the vertical axis is thedetection channel output SNR (after homodyne recombination, wherenecessary, and resampling).

Curve 1702 represents ordinary detection of a simulated hologram usingASK. Curve 1704 represents the same hologram detected with an ideallocal oscillator that is phase matched in time and space with the phasecarrier of the reconstructed data beam, with power 100 times that of thehologram. Curve 1706 illustrates a PSK modulated hologram with an ideallocal oscillator that is phase matched in time and space with the phasecarrier of the reconstructed data beam. Curve 1708 illustrates a curvefor quadrature homodyne detection, as described herein, of a detectedPSK modulated hologram, such as the one discussed above with referenceto FIGS. 5, 6, and 10. As can be seen in FIG. 17, for a given requiredoutput SNR (say, 2 dB), the ASK homodyne detector may tolerate about 2dB more input noise, and the PSK curves may tolerate about 3 dB beyondthat. It should be noted that these curves shown in FIG. 17 aresimulated curves. Although the normal detection curve 1702 in FIG. 17eventually catches up to the homodyne curves 1704, 1706 and 1708 at highSNR, this may be due to the resampling method in the simulation, and notthe regime that a real system would operate in. As shown in FIG. 17, thequadrature homodyne curve 1708 closely tracks the ideal PSK homodynecurve until very high levels of noise start to degrade the quality ofthe P and Q phase carrier estimates.

Thus, PSK quadrature homodyne may provide a ˜5 dB of improvementcompared to normal detection, while at the same time increasing thepotential transfer rate (by decreasing exposure times) and equalizingthe holographic write illumination (e.g., eliminating the D.C. hotspot). Using PSK quadrature homodyne detection may ultimately permitgreater storage densities by permitting many weaker holograms to bestored in a recoverable fashion.

In addition to homodyne detection, embodiments of the present inventionmay also be used for or with heterodyne detection. For heterodynedetection, the local oscillator beam may have a phase carrier with adifferent frequency than the phase carrier of the reconstructed databeam. Heterodyne detection may be accomplished, for example, byfrequency shifting the local oscillator beam using an acousto-opticmodulator, in place of the switchable quarter wave plate. In the case ofheterodyne detection, the intensity of each detected pixel may bemodulated in time at a rate equal to the frequency difference betweenthe reconstructed data beam and the local oscillator beam, and the phaseof this modulated combined beam will be determined by the phasedifference between the reconstructed data beam and the local oscillatorbeam. A processor may capture enough of the modulated waveforms (˜onecycle) to determine their amplitude and phase. By then applying thecross-correlation peak detection method discussed above with referenceto FIG. 7 at some reference time-slice within the page-wide waveform,the processor may determine the absolute phase difference with respectto the local oscillator at each pixel position, and thus extract thedata from the detected signal by conventional heterodyne processing.

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

Although the present invention has been fully described in conjunctionwith several embodiments thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. (canceled)
 2. (canceled)
 3. A method for use with a storage mediumthat holographically stores information, the method comprising thefollowing steps: (a) generating a reconstructed data beam by directing afirst coherent light beam into a storage medium, wherein the firstcoherent light beam reproduces a reference beam used to holographicallystore information in the storage medium; (b) obtaining a first imagefrom a first combined beam produced by combining at least a portion ofthe reconstructed data beam with at least a portion of the secondcoherent light beam, wherein the reconstructed data beam and the secondcoherent light beam have a phase difference that is a first phasedifference; (c) obtaining a second image from a second combined beamproduced by combining the reconstructed data beam with the secondcoherent light beam, wherein the phase difference of the reconstructeddata beam and the second coherent light beam has been adjusted toproduce a second phase difference; (d) processing the first image andsecond image to obtain first image contrast information and second imagecontrast information; (e) obtaining combined information from the firstimage and second image using the first image contrast information andthe second image contrast information; and (f) processing the combinedinformation to obtain the information holographically stored by thestorage medium; wherein the holographic storage medium stores a hologramof a data page comprising a plurality of pixels, wherein the data pagecomprises a plurality of reserved blocks, wherein each reserved blockcomprises a known pixel pattern, and wherein processing the first imageand the second image to obtain first image contrast information andsecond image contrast information comprises: determining informationregarding a contrast for at least one of the plurality of reservedblocks; wherein processing the first image and the second image toobtain first image contrast information and second image contrastinformation further comprises: interpolating a contrast for a pixelusing the information determined regarding a contrast for at least oneof the plurality of reserved blocks.
 4. (canceled)
 5. (canceled) 6.(canceled)
 7. (canceled)
 8. The method of claim 3, wherein theholographic memory system uses off-axis holography.
 9. The method ofclaim 3, wherein the holographic memory system uses collinearholography.
 10. The method of claim 3, wherein the first phasedifference and the second phase difference have a phase difference of 90degrees.
 11. The method of claim 3, wherein combining information fromthe first image and second image comprises: multiplying a filteredintensity level for a pixel of the first image by a first factordetermined by the contrast levels for the pixels of the first image andsecond image to obtain a first adjusted value; multiplying a filteredintensity level for a pixel of the second image by a second factordetermined by the contrast levels for the pixels of the first image andsecond image to obtain a second adjusted value, wherein the pixel forthe first image and the pixel for the second image correspond to thesame pixel of a data page holographically stored by the storage medium;and determining an adjusted intensity level for the data page pixel bysumming the first adjusted value and the second adjusted value.
 12. Themethod of claim 11, further comprising: determining an adjustedintensity level for each pixel of the data page by, for each pixel inthe data page, performing the steps of multiplying and determining anadjusted intensity level for the pixel.
 13. (canceled)
 14. (canceled)15. A system for use with a storage medium that holographically storesinformation, the system comprising: a light source which generates afirst coherent light beam that is a reproduction of a reference beamused in holographically storing information in a storage medium andwherein the first coherent beam generates a reconstructed data beam fromthe storage medium; a beam combiner which combines at least a portion ofthe reconstructed data beam with at least a portion of a second coherentlight beam to produce a combined beam; a camera which captures an imagefrom the combined beam; a phase retarder which causes a first phasedifference and a second phase difference between the reconstructed databeam and the second coherent beam; and a processor which (a) directs thephase retarder to cause the first phase difference between thereconstructed data beam and the second coherent beam to obtain a firstimage from the camera of the combined beam when the reconstructed databeam and the second coherent beam have the first phase difference; (b)directs the phase retarder to cause the second phase difference betweenthe reconstructed data beam and the second coherent beam to obtain asecond image from the camera of the combined beam when the reconstructeddata beam and the second coherent beam have the second phase difference;(c) processes the first image and second image to obtain first imagecontrast information and second image contrast information, (d) combinesinformation from the first image and second image using the first imagecontrast information and the second image contrast information to obtaincombined information; and (e) processes the combined information toobtain the information holographically stored by the storage medium;wherein the holographic storage medium stores a hologram of a data pagecomprising a plurality of pixels and wherein the data page comprises aplurality of reserved blocks, wherein each reserved block comprises aknown pixel pattern, and wherein the processor in processing the firstimage and second image to obtain first image contrast information andsecond image contrast information is further configured to determineinformation regarding a contrast for at least one of the plurality ofreserved blocks, and wherein the processor, in processing the firstimage and the second image to obtain first image contrast informationand second image contrast information, is further configured tointerpolate a contrast for a pixel using the information determinedregarding a contrast for at least one of the plurality of reservedblocks.
 16. (canceled)
 17. (canceled)
 18. (canceled)
 19. (canceled) 20.The system of claim 15, wherein the holographic memory system usesoff-axis holography.
 21. The system of claim 15, wherein the holographicmemory system uses collinear holography.
 22. The system of claim 15,wherein the first phase difference and the second phase difference havea phase difference of 90 degrees.
 23. The system of claim 15, whereinthe processor in obtaining combined information from the first image andsecond image using the first image contrast information and the secondimage contrast information is further configured to: multiply a filteredintensity level for a pixel of the first image by a first factordetermined by the contrast levels for the pixels of the first image andsecond image to obtain a first adjusted value; multiply a filteredintensity level for a pixel of the second image by a second factordetermined by the contrast levels for the pixels of the first image andsecond image to obtain a second adjusted value, wherein the pixel forthe first image and the pixel for the second image correspond to thesame pixel of a data page holographically stored by the storage medium;and determine an adjusted intensity level for the data page pixel bysumming the first adjusted value and the second adjusted value.
 24. Thesystem of claim 23, where the processor is further configured to:determine an adjusted intensity level for each pixel of the data pageby, for each pixel in the data page, performing the steps of multiplyingand determining an adjusted intensity level for the pixel.
 25. Thesystem of claim 24, wherein the phase retarder is a ferroelectricquarter waveplate.
 26. The system of claim 14, wherein the beam combineris a non-polarizing beam splitter.
 27. The system of claim 14, whereinthe camera is a CMOS detector array or a charged coupled device. 28.(canceled)
 29. (canceled)
 30. A system for use with a storage mediumthat holographically stores information, the system comprising: meansfor generating a reconstructed data beam by directing a first coherentlight beam into a storage medium, wherein the first coherent light beamreproduces a reference beam used to holographically store information inthe storage medium; means for obtaining a first image from a firstcombined beam produced by combining at least a portion of thereconstructed data beam with at least a portion of a second coherentlight beam, wherein the reconstructed data beam and the second coherentlight beam have a phase difference that is a first phase difference;means for obtaining a second image from a second combined beam producedby combining the reconstructed data beam with the second coherent lightbeam, wherein the phase difference of the reconstructed data beam andthe second coherent light beam has been adjusted to produce a secondphase difference; means for processing the first image and second imageto obtain first image contrast information and second image contrastinformation; and means for obtaining combined information from the firstimage and second image using the first image contrast information andthe second image contrast information; and means for processing thecombined information to obtain the information holographically stored bythe storage medium; wherein the holographic storage medium stores ahologram of a data page comprising a plurality of pixels, wherein thedata page comprises a plurality of reserved blocks, wherein eachreserved block comprises a known pixel pattern, and wherein the meansfor processing the first image and the second image to obtain firstimage contrast information and second image contrast informationcomprises: means for determining information regarding a contrast for atleast one of the plurality of reserved blocks; and wherein the means forprocessing the first image and the second image to obtain first imagecontrast information and second image contrast information furthercomprises: means for interpolating a contrast for a pixel using theinformation determined regarding a contrast for at least one of theplurality of reserved blocks.
 31. (canceled)
 32. (canceled) 33.(canceled)
 34. (canceled)
 35. The system of claim 30, wherein theholographic memory system uses off-axis holography.
 36. The system ofclaim 30, wherein the holographic memory system uses collinearholography.
 37. The system of claim 30, wherein the first phasedifference and the second phase difference have a phase difference of 90degrees.
 38. The system of claim 30, wherein the means for combininginformation from the first image and second image comprises: means formultiplying a filtered intensity level for a pixel of the first image bya first factor determined by the contrast levels for the pixels of thefirst image and second image to obtain a first adjusted value; means formultiplying a filtered intensity level for a pixel of the second imageby a second factor determined by the contrast levels for the pixels ofthe first image and second image to obtain a second adjusted value,wherein the pixel for the first image and the pixel for the second imagecorrespond to the same pixel of a data page holographically stored bythe storage medium; and means for determining an adjusted intensitylevel for the data page pixel by summing the first adjusted value andthe second adjusted value.
 39. The system of claim 38, furthercomprising: means for determining an adjusted intensity level for eachpixel of the data page by, for each pixel in the data page, performingthe steps of multiplying and determining an adjusted intensity level forthe pixel.
 40. A method for use with a storage medium thatholographically stores information, the method comprising the followingsteps: (a) generating a combined beam comprising a local oscillatorportion and a reference pattern portion; (b) causing the combined beamto contact the storage medium to thereby generate a reconstructed databeam; (c) obtaining an image from the reconstructed data beam; (d)processing the obtained image to provide image contrast information; and(e) modifying the local oscillator portion of the combined beam based onthe image contrast information.
 41. The method of claim 40, wherein thecombined beam comprising a local oscillator portion and a referencepattern portion is formed using a format where a first portion of theformat is used for forming the local oscillator portion and a secondportion of the format is used for forming the reference pattern portion.42. The method of claim 41, wherein in generating the combined beam, thereference pattern portion comprises a reference pattern corresponding toa reference pattern used in holographically storing the information. 43.The method of claim 41, wherein modifying the local oscillator portioncomprises: modifying at least one pixel in the local oscillator portionof the reference pattern based on the determined contrast determined forthe at least one pixel.
 44. The method of claim 42, wherein modifyingthe at least one pixel comprises: directing the at least one pixel toeffect a phase shift in at least a portion of the combined beam based onthe determined contrast for the at least one pixel.
 45. The method ofclaim 39, wherein the holographic storage medium stores a hologram of adata page comprising a plurality of pixels and wherein the data pagecomprises a plurality of reserved blocks, wherein each reserved blockcomprises a known pixel pattern, wherein processing the obtained imageto provide image contrast information further comprises: determininginformation regarding a contrast for at least one of the plurality ofreserved blocks.
 46. The method of claim 45, wherein processing theobtained image to provide image contrast information further comprises:interpolating a contrast for a pixel not included in a reserved blockusing the information determined regarding a contrast for at least oneof the plurality of reserved blocks.
 47. The method of claim 45, whereindetermining information regarding a contrast for at least one of theplurality of reserved blocks comprises: calculating a cross-correlation,using an expected pattern, over a region comprising a subset of pixelsof the data page.
 48. A system for use with a storage medium thatholographically stores information, the system comprising: a lightsource generating a first coherent light beam which reproduces areference beam used in holographically storing information in a storagemedium; a spatial light modulator (SLM) which forms a combined beam fromthe first coherent light beam, wherein the SLM comprises a plurality ofpixels which implement a format, wherein the format comprises a localoscillator portion and a reference pattern portion; an optical steeringsubsystem which directs the first coherent light beam towards thestorage medium to generate a reconstructed data beam; a camera whichobtains an image from the reconstructed data beam; a processor whichprocesses the obtained image to provide image contrast information andto direct the SLM to modify at least one pixel in the local oscillatorportion based on the image contrast information.
 49. The system of claim48, wherein in generating the combined beam, the reference patternportion comprises a reference pattern corresponding to a referencepattern used in holographically storing the information.
 50. The systemof claim 48 wherein the SLM is configured to effect a phase shift in atleast a portion of the combined beam in response to an instructionreceived from the processor.
 51. The system of claim 48, wherein theholographic storage medium stores a hologram of a data page comprising aplurality of pixels and wherein the data page comprises a plurality ofreserved blocks, wherein each reserved block comprises a known pixelpattern, and wherein the processor in processing the obtained image toprovide image contrast information is further configured to: determineinformation regarding a contrast for at least one of the plurality ofreserved blocks.
 52. The system of claim 51, and wherein the processorin processing the obtained image to provide image contrast informationis further configured to: interpolate a contrast for a pixel notincluded in a reserved block using the information determined regardinga contrast for at least one of the plurality of reserved blocks.
 53. Thesystem of claim 51, wherein the processor in determining informationregarding a contrast for at least one of the plurality of reservedblocks is further configured to: calculate a cross-correlation, using anexpected pattern, over a region comprising a subset of pixels of thedata page.
 54. A system for use with a storage medium thatholographically stores information, the system comprising: means forgenerating a combined beam comprising a local oscillator portion and areference pattern portion; means for causing the combined beam tocontact the storage medium to thereby generate a reconstructed databeam; means for obtaining an image from the reconstructed data beam;means for processing the obtained image to provide image contrastinformation; and means for modifying the local oscillator portion of thecombined beam based on the image contrast information.
 55. The system ofclaim 54, wherein the combined beam comprising a local oscillatorportion and a reference pattern portion is formed using a format where afirst portion of the format is used for forming the local oscillatorportion and a second portion of the format is used for forming thereference pattern portion.
 56. The system of claim 55, wherein ingenerating the combined beam, the reference pattern portion comprises areference pattern corresponding to a reference pattern used inholographically storing the information.
 57. The system of claim 54,wherein the means for modifying the local oscillator portion, comprises:means for modifying at least one pixel in the local oscillator portionof the reference pattern based on the determined contrast determined forthe at least one pixel.
 58. The system of claim 57, wherein the meansfor modifying the at least one pixel, comprises: means for directing theat least one pixel to effect a phase shift in at least a portion of thecombined beam based on the determined contrast for the at least onepixel.
 59. The system of claim 54, wherein the holographic storagemedium stores a hologram of a data page comprising a plurality of pixelsand wherein the data page comprises a plurality of reserved blocks,wherein each reserved block comprises a known pixel pattern, wherein themeans for processing the obtained image to provide image contrastinformation comprises: means for determining information regarding acontrast for at least one of the plurality of reserved blocks.
 60. Thesystem of claim 59, and wherein the means for processing the obtainedimage to obtain contrast information further comprises: means forinterpolating a contrast for a pixel not included in a reserved blockusing the information determined regarding a contrast for at least oneof the plurality of reserved blocks.
 61. The system of claim 59, whereinthe means for determining information regarding a contrast for at leastone of the plurality of reserved blocks comprises: means for calculatinga cross-correlation, using an expected pattern, over a region comprisinga subset of pixels of the data page.
 62. A method for use with a storagemedium that holographically stores information, the method comprisingthe following steps: (a) generating a reconstructed data beam bydirecting a first coherent light beam into a storage medium, wherein thefirst coherent light beam reproduces a reference beam used toholographically store information in the storage medium; (b) generatinga local oscillator beam comprising a phase modulation pattern; (c)generating a combined beam comprising at least a portion of the localoscillator beam and at least a portion of the reconstructed data beam;(d) obtaining an image from the combined beam; (e) processing theobtained image to provide image contrast information; and (f) modifyingthe phase modulation pattern of the local oscillator beam based on theimage contrast information.
 63. The method of claim 62, whereingenerating a combined beam comprises: reflecting at least a portion ofthe local oscillator beam by a beam combiner; and directing thereconstructed data beam to the beam combiner such that at least aportion of the reconstructed data beam passes through the beam combinerand combines with the portion of the local oscillator beam reflected bythe beam combiner.
 64. The method of claim 63, wherein the beam combinercomprises a partially reflective surface.
 65. The method of claim 64,wherein the partially reflective surface is in a fourier plane or animage plane of the phase modulation pattern.
 66. The method of claim 62,wherein generating a local oscillator beam comprising a phase modulationpattern, comprises: directing the local oscillator beam towards aspatial light modulator (SLM) which imparts the local oscillator beamwith a desired phase modulation pattern.
 67. The method of claim 62,wherein the holographic storage medium stores a hologram of a data pagecomprising a plurality of pixels and wherein the data page comprises aplurality of reserved blocks, wherein each reserved block comprises aknown pixel pattern, and wherein processing the obtained image toprovide image contrast information further comprises: determininginformation regarding a contrast for at least one of the plurality ofreserved blocks.
 68. The method of claim 67, and wherein processing theobtained image to provide image contrast information further comprises:interpolating a contrast for a pixel not included in a reserved blockusing the information determined regarding a contrast for at least oneof the plurality of reserved blocks.
 69. The method of claim 67, whereindetermining information regarding a contrast for at least one of theplurality of reserved blocks comprises: calculating a cross-correlation,using an expected pattern, over a region comprising a subset of pixelsof the data page.
 70. A system for use with a storage medium thatholographically stores information, the system comprising: a lightsource generating a first coherent light beam which reproduces areference beam used in holographically storing information in a storagemedium and wherein the first coherent beam generates a reconstructeddata beam from the storage medium; an SLM which imparts a phasemodulation pattern to a second coherent beam; a beam combiner whichcombines at least a portion of the reconstructed data beam with at leasta portion of a second coherent beam; a camera which obtains an imagefrom the combined beam; and a processor which processes the obtainedimage to provide image contrast information and to direct the SLM tomodify the phase modulation pattern based on the image contrastinformation.
 71. The system of claim 70, wherein the beam combinercomprises: a beam combiner which forms a combined beam by reflecting atleast a portion of the second coherent beam which combines with at leasta portion of the reconstructed data beam which passes through the beamcombiner.
 72. The system of claim 71, wherein the beam combinercomprises a partially reflective surface is in a fourier plane or animage plane of the phase modulation pattern.
 73. The system of claim 70,wherein the holographic storage medium stores a hologram of a data pagecomprising a plurality of pixels and wherein the data page comprises aplurality of reserved blocks, wherein each reserved block comprises aknown pixel pattern, and wherein the processor in processing theobtained image to provide image contrast information is furtherconfigured to: determine information regarding a contrast for at leastone of the plurality of reserved blocks.
 74. The system of claim 73,wherein the processor in processing the obtained image to provide imagecontrast information is further configured to interpolate a contrast fora pixel not included in a reserved block using the informationdetermined regarding a contrast for at least one of the plurality ofreserved blocks.
 75. The system of claim 73, wherein the processor indetermining information regarding a contrast for at least one of theplurality of reserved blocks is further configured to: calculate across-correlation, using an expected pattern, over a region comprising asubset of pixels of the data page.
 76. A system for use with a storagemedium that holographically stores information, the system comprising:means for generating a reconstructed data beam by directing a firstcoherent light beam into a storage medium, wherein the first coherentlight beam reproduces a reference beam used to holographically storeinformation in the storage medium; means for generating a localoscillator beam comprising a phase modulation pattern; means forgenerating a combined beam comprising at least a portion of the localoscillator beam and at least a portion of the reconstructed data beam;means for obtaining an image from the combined beam; means forprocessing the obtained image to provide image contrast information; andmeans for modifying the phase modulation pattern of the local oscillatorbeam based on the image contrast information.
 77. The system of claim76, wherein the means for generating a combined beam comprises: meansfor reflecting at least a portion of the local oscillator beam by a beamcombiner; and means for directing the reconstructed data beam thepolytopic aperture such that at least a portion of the reconstructeddata beam passes through the polytopic aperture and combines with theportion of the local oscillator beam reflected by the polytopicaperture.
 78. The system of claim 77, wherein the beam combinercomprises a partially reflective surface.
 79. The system of claim 78,wherein the partially reflective surface is in a fourier plane or animage plane of the phase modulation pattern.
 80. The system of claim 77,wherein the means for generating a local oscillator beam comprising aphase modulation pattern, comprises: means for directing the localoscillator beam towards a spatial light modulator (SLM) which impartsthe local oscillator beam with a desired phase modulation pattern. 81.The system of claim 76, wherein the holographic storage medium stores ahologram of a data page comprising a plurality of pixels and wherein thedata page comprises a plurality of reserved blocks, wherein eachreserved block comprises a known pixel pattern, wherein the means forprocessing the obtained image to provide image contrast informationfurther comprises: means for determining information regarding acontrast for at least one of the plurality of reserved blocks.
 82. Thesystem of claim 81, wherein the means for processing the obtained imageto provide image contrast information further comprises: means forinterpolating a contrast for a pixel not included in a reserved blockusing the information determined regarding a contrast for at least oneof the plurality of reserved blocks.
 83. The system of claim 81, whereinthe means for determining information regarding a contrast for at leastone of the plurality of reserved blocks comprises: means for calculatinga cross-correlation, using an expected pattern, over a region comprisinga subset of pixels of the data page.